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From 2016 s4 patch 2 release notes:
- Aston Martin DBR9 GT1, Chevrolet Corvette C6.R GT1, Ford GT
Tire warmers are now used for this car. Temperature is set to what we
would see with blankets or an oven, as used on GT3 cars.

- Dallara DW12

Road course tire wear has been significantly increased to ensure more realistic degradation.

From 2016 s3 release notes:
We
have performed an extensive update to all track surfaces to better
model physics collisions, even with tiny particles like sparks, gravel,
and marbles.
The dynamic
track surface now includes dust and gravel, which is brought onto the
racing surface from off-track excursions and affects tire grip.

-
Dust and gravel can now be dragged and thrown onto the track by wayward
cars, which then interacts with the tires to affect grip and
accumulation of debris on the tires.

-
The way the Dynamic Track is stored in a replay has been improved to
allow sudden changes in the track surface state to be seen immediately.

Aston Martin DBR9 GT1
- The tire compound has been adjusted to better handle high operating temperatures; it now is matched to the Chevrolet Corvette C6.R GT1.

Cadillac CTS-V Racecar
- Tire construction has been changed to be more forgiving at and over the limit.

Chevrolet Corvette C6.R GT1
- A more temperature resistant tire compound has been added, so that the Chevrolet Corvette C6.R GT1 is more competitive with the Aston Martin DBR9 GT1, particularly in hot ambient conditions.

Ford Falcon FG V8
(see release notes for big changes)
- This vehicle now uses V6 tires. These tires are capable of maintaining grip at higher operating temperatures, so they should feel less greasy in hot ambient conditions and more catchable in big slides. However, be aware that grip will be slower to build when the tires are cold, so it will feel icy for the first few laps when ambient temperatures are cold. Also, you will nominally find peak grip at around the 3rd to 5th timed lap on a qualifying run. We understand this is not ideal for 2-lap qualifying runs, but felt that better grip at high operating temperatures (where the tires run most of the time) would be a worthwhile tradeoff against a relatively slow build of grip when cold.

Holden Commodore VF V8
(see release notes for big changes)
- This vehicle now uses V6 tires. These tires are capable of maintaining grip at higher operating temperatures, so they should feel less greasy in hot ambient conditions and more catchable in big slides. However, be aware that grip will be slower to build when the tires are cold, so it will feel icy for the first few laps when ambient temperatures are cold. Also, you will nominally find peak grip at around the 3rd to 5th timed lap on a qualifying run. We understand this is not ideal for 2-lap qualifying runs, but felt that better grip at high operating temperatures (where the tires run most of the time) would be a worthwhile tradeoff against a relatively slow build of grip when cold.

Radical SR8
- Unfortunately, the developments for Season 2 did not have the desired effects on car performance, drive-ability, and real-world correlation. As a result, this vehicle has been restored to the 2016 Season 1 specifications.

From 2016 s2 patch 2.1 release notes:TIRES
- Fixed a bug where performing a driver swap would not transfer how cured the tire's rubber was to the next driver, so the next driver would be able to continue on much faster rubber without having taken the time to swap a tire.
-- Please note that a separate existing bug, where the reported values in the Tire Info's Black Box may not show the correct values during a driver swap, has not been fixed at this time, but you may be assured that the physics and racing behavior are working correctly.

From 2016 s2 patch 1 release notes:
BLACK BOXES
-
Setup changes that are applied during a pit stop (like tire pressures)
are no longer kept when the Sim advances from one session to another.
Immediate in-cockpit adjustments (like brake bias or traction control
setting) are still kept across a session transition.

Initial temperatures of wheels and tires have been improved, especially for cars with tire warmers.
Previously, the entire wheel and tire was heated to the tire warmer temperature; now only the tire surface is heated to the specified temperature, while the wheel is more appropriately closer to the ambient temperature. This may produce a subtle change in ride height in the garage, so cars with tire warmers may need an adjustment to pass Tech if they were close to the limit on ride height.

OPPONENT CARS

- Smoke and debris generated by opponent vehicles can now be seen.

- Opponent vehicles now show wheelspin and lockup, and skids and smoke from these actions are more realistic.

DYNAMIC TIRE (TYRE) DEBRIS ACCUMULATION

Tires on all vehicles now visually show the accumulation and dispersal of debris. This includes materials such as grass, gravel, rubber marbles, dust, and sand. These materials are picked up when driven over based on the surface type, grip, and speed of the vehicle, and then they are removed over time by the same factors. The physics model for this activity has already been running since last season's update, so there is no change to driving characteristics, but now you can see the effects!

TIRE / TYRE UPDATES
see release notes for much more detail

BMW Z4 GT3, converted to Pirelli. Updated to match Mercedes AMG GT3.
Chevrolet Monte Carlo SS, reduced tire grip.
McLaren MP4-12c GT3, converted to Pirelli.
McLaren MP4-30, tire wear and degradation rate has been increased.
NASCAR - check the release-notes for various things.
Radical SR8, tire grip has been increased.
Star Mazda, tires now heat up faster; the goal is to have the tires come in more quickly from cold. Ideally, a fast lap is a timed 3rd or 4th lap.
Star Mazda, tire stiffness has been greatly reduced at both front and rear. Tires now seem better matched and have slightly more progressive breakaway characteristics.

Cars with tire warmers will now use the tire warmers while in the garage. The "Cold Pressure" setup item is now renamed "Starting Pressure"
to reflect this change. iRacing setups have been adjusted for this
change, but you will need to increase your tire pressure on any
pre-existing setups to keep your hot tire pressures out on the track in
the right place. You will need to raise your cold pressures for the
various cars that have tire warmers by approximately the following
amounts in order to convert them to a "Starting Pressure" value:

Aston Martin DBR9 GT1 = +3.0 psi

BMW Z4 GT3 = +6.5 psi

Chevrolet Corvette C6.R GT1 = +3.0 psi

Ford GT GT3 = +6.5 psi

HPD ARX-01c = +5.0 psi

McLaren MP4-12C GT3 = +6.5 psi

Ruf RT 12R Track = +6.5 psi

Williams-Toyota FW31 = + 5.0 psi.

-
Cars without tire warmers will now enter the world with exactly the
tire pressure specified in the garage, instead of always being a couple
of psi higher than specified. iRacing setups have been adjusted for this
change, but you will need to increase your tire pressure on any
pre-existing setups to keep your hot tire pressures out on the track in
the right place. The typical change for any car will be approximately
+1.5 psi.

McLaren MP4-30
-
New car added! The McLaren MP4-30 Formula 1 racecar features a few
cutting edge systems that help set it apart from the pack, including the
Drag Reduction System (DRS) to enable boosts of speed on straights, the
Energy Recovery System (ERS) to re-charge the vehicle's batteries, and
the Motor Generator Unit (MGU) to provide even more power for
overtaking. Jump in the cockpit and get ready to experience a truly
modern racing machine.

• GARAGE TEMPS post by Eric H (16 Dec 2015):
Just
for clarification our 'garage temperatures' are a combination of
carcass and surface temps using conduction methods to calculate what is
displayed.
They are not solely surface temps, which you can get from telemetry, nor are they carcass temps.
This method is how a probe would read; remember a tire probe pierces the surface and takes a reading within the tread.

In
regard to flat spotting we don't have a model yet that localizes rapid
heating that would have blocks of the tread that are hotter than others.
If you lock a wheel your entire tire surface 'gets hot'. This is partly why too much grip is lost in those types of events.

There
is somewhat of a small bug in our implementation of tire warmers in
that the blanket is essentially also heating up the wheels!
The
carcass temps fall because wheels cool fairly quickly, which therefore
bring down the tire pressures due to conduction through the rims.
If the wheels were kept at ambient instead there would definitely be less heat flow out while driving hard.
We're working on a fix. At this point it just means you may have to apply more 'starting pressure' to hit your on-track targets.

• Post by Eric H (18 Dec 2015):
We could output temps in any form or fashion - and for dev purposes we do.
But what we are trying to do for members is output temperatures that are representative of how they would be measured on a real car at the race track.
Those being surface temps (akin to using IR mounted sensors), core temps (akin to using a pyrometer probe) and at least on one other car inner air temperature (akin to an internal air temp sensor).

BTW, there is no funny business going on trying to hide how the tire model works.
It's pretty organic and would be hard for us to do something like that anyway - every bit is linked to every other bit and hacks would break the thing!
The reason we don't output a lot more information isn't for any maniacal theory but because outputting things that aren't physically representative in the real world doesn't really make a lot of sense.
For instance, you can't measure carcass temps on a real car either on pit road or on the race track.
(See thread for more comments).

• Post on the OVAL side by Eric H (18 Dec 2015):
1. Less tread changes the cornering stiffness of the tire.
This change doesn't have anything to do with the carcass and it's stiffness but really the amount of grip per unit of slip.
Increasing cornering stiffness of a tire, say by shaving it, can make it feel more edgy, have a higher grip peak but also a harder grip fall off beyond peak.
2. Optimum range is as cool as you can possibly keep them.
You'll notice your early laps are fastest and that's because they are coolest and therefore have the most grip.
3. I can't answer that because it'll depend on all sorts of things. Generally, though, you should target less spread than more spread.
4. I'm not clear on this question but I can say that the tires do deform in all directions - this includes the carcass sidewall, carcass belts and the tread.
5. It's not a percentage per se but a heat flow calculation based on how deep we are measuring. That'll depend on how much tread is remaining. Then once that's figured we'll calculate the heat flux. If the surface is really really hot and the carcass is cool more of the temp reading will come from the surface. Vice versa is true too.
From the same thread...
Q: Ok, how does the cornering stiffness go UP with LESS material on the surface?
A: Less tread will shear proportionally less.

Reference to shear explained by Eric H, post:
You're using shear as a verb and I'm referring to it as a noun. I could have used the term deflection. A tire with half the tread, notwithstanding the improved thermal control, will see half the deflection for a given load. To reach equilibrium and generate the same force more tread depth will require more deflection, or slip.

Another analogy for car guys out there. Cut a spring in half and it's stiffness will double. The material properties haven't changed but the resultant rate of deflection has. A tire tread 'spring' in shear, bending, compression, or whatever works the same way.

But, a big but, as tread wears down it can effectively get harder as the tread depth approaches zero. The reason for this is the really stiff carcass can now impart some of it's stiffness into the rubber. Think of a huge soft block of rubber sitting on your desk in front of you, like 5 inches thick or something. If you press your fingers into it perhaps it will deflect a bunch. Now shave a very thin layer off of it and try to deflect that very thin layer. It won't deflect nearly as much because of the solid surface underneath, being your desk, is limiting it. Same thing can happen on a tire. But we're talking at less than probably one to half a millimeter, or something, of tread remaining.

Noted: Don't get confused with a stiffer construction and a stiffer grip response. Interlinked they are but they are also two different things. Tire heating affects performance from a cycling standpoint too. Tires get harder as they heat up. RS tires will heat more than LS tires on an oval car and harden faster.

Related post by Eric H: "Really tires have some amount of stiffness even when air pressure is zero.
Although
our model hasn't been going to zero stiffness when a tire contains no
air DK has come up with a better method to calculate how stiff the
carcass behaves when it this condition."

Another related post by Eric H: "It
does matter now as this change impacts the stiffness slope with change
in air pressure. Off of some reference inflation pressure less air
pressure will retain more stiffness and more air pressure will gain less
than before."

2015s4 release notes:
V6
tires now include a term in the lateral stiffness calculations that was
missing in the NTM previously (including the v1-5 tires).
All prior cars with v6 tires from last season have been revised to suit, and many more cars have been upgrade to v6 tires.

V6 tyre changes: driveability, tyre degradation, and vehicle response.
2015s3 release notes:
Dave K has a new model of the tire carcass that does a much more accurate computation of the tire's stiffness.
Amongst other things tires now respond much more correctly to tire pressure changes.
This is what is called a "Version 6" tire. Most of our road course cars have been updated with this.